I had a P99/AS, and a P99/AS.. Both were excellent pistols.. My buddy was so impressed with my P99/AS that he tried to buy one for himself (this was a couple of years ago), but they were few and far between so he bought a new SW99..
He and I shot our P99/SW99's head to head more than once, and could discern no appreciable difference in performance either as to accuracy or reliability..
FWIW, the P99 as imported by S&W is warranted by S&W for one year.. The SW99 comes with a LIFETIME warranty, again by S&W.
I'd recommend the SW99 in a heartbeat, but I've heard not so good things about the SW990, altho I have no personal experience with that model.